Structure of miniature X-ray source

ABSTRACT

Miniature X-ray source comprising a support structure provided with a through-going hole, an anode is arranged at one end and a cathode ( 8,24 ) at the other end of the hole, thereby defining a cavity, the anode and cathode are adapted to be energised in order to generate X-ray radiation. The support structure has a cross-sectional shape that is determined such that a desired radiation distribution of the radiation generated by the X-ray source is achieved. Also a method of manufacturing miniature X-ray sources is disclosed.

FIELD OF THE INVENTION

[0001] The present invention relates to a miniature X-ray source and toa method of manufacturing miniature X-ray sources according to thepreambles of the independent claims.

BACKGROUND OF THE INVENTION

[0002] In treating stenosis in coronary arteries, a restenosis occurs in30-60% of the cases. It is known that a treatment with beta- or gamma-(X-ray) radiation will decrease the occurrence of restenosissubstantially.

[0003] Another example of an application of the present invention istreatment of cancer tumors where it is desired to deliver radiationlocally.

[0004] Methods to apply the radiation to the site of treatment arepresently subject to intensive research Generally a hollow catheter isinserted into the body, typically via an artery, in such a way that itsdistal end is placed near the site of treatment. A source of radiationattached to the distal end of an elongated member is inserted into thehollow catheter, and is forwarded until the radiation source is disposedat a proper position for radiating the site of treatment. In thespecific case of treating cardiac vessels, the catheter is placed nearthe cardiac vessel tree (this catheter often called a “guide catheter”).A very thin wire—called guide wire—is then used to probe further andreach the site where treatment shall be performed. The therapeuticdevice is moved along this wire, i.e. by threading the device onto theguide wire. It is obvious that the therapeutic device has to have a holeclose to its distal end in order to do this.

[0005] Radiation treatment methods using radioactive pellets or balloonsetc. as radiation source is known in the art. Since these methods havesome drawbacks, such as the need for substantial efforts to controlradiation in the environment outside the patient, the use of a miniatureelectrical X-ray source including a cold cathode has been proposed. Sucha source may be switched on and off due to its electrical activation. Anexample of such an X-ray source is described in the U.S. Pat. No.5,854,822 as well as in U.S. Pat. No. 5,984,853.

[0006] U.S. Pat. No. 5,984,853 discloses a method and apparatus ofcreating a miniaturized source of radiation and delivering radiation toa location such as therapy location. The radiation source is built upfrom two plates with a recessed region forming a microcavity at one orseveral localities. An anode material and a cathode with extremely smalldimensions, and having the form of a sharp tip are located within thismicrocavity. During the manufacturing process lithographic and etchingtechniques according to well-known techniques are used to define thestructures of the microcavity, the anode and the cathode By using theabove-mentioned fabrication techniques the manufacturing cost per unitbecomes very small when the elements are fabricated in large numbers.This is due to the fact that batch fabrication with thousands of unitsper batch is feasible.

[0007] However, the apparatus disclosed in U.S. Pat. No. 5,984,853 doesnot take into account the spatial distribution of the generatedradiation.

[0008] One object of the present invention is to achieve a structure ofX-ray source allowing manufacturing of a large number of X-ray sourcesthat fulfills requirements regarding radiation distribution of thegenerated X-ray radiation

SUMMARY OF THE INVENTION

[0009] The above-mentioned objects are achieved by a miniature X-raysource and a method of manufacturing miniature X-ray sources accordingto the independent claims.

[0010] Preferred embodiments are set forth in the dependent claims.

[0011] Conventionally in the semiconductor technology the individualchips obtain a square shape, since it is the most efficient way ofcutting (sawing) the wafer, and in addition the wafer is optimallyutilized in this way, since no waste being produced.

[0012] In order to be able to produce a large number of miniature X-raysources for the above mentioned type of applications the production isconveniently made in batch processes, starting with a disc-shaped waferhaving a diameter of e.g. 4″ of a suitable material. Obviously the wafermay also be square or rectangular or polygonal in its shape. By usingvarious techniques known per se from the semiconductor technology, suchas lithography combined with etching and deposition techniques, a largenumber of discrete components can be made from one wafer. Finally, eachindividual component is cut out from the wafer by e.g. a sawingoperation or by laser etching. Other known methods include sawing,blasting end using scribe lines to crack the wafer to discrete parts.

[0013] In X-ray radiation therapy inside a living body, and inparticular in blood vessels that have a tubular shape, i.e. a circularsymmetry, it is desirable that the delivered radiation is uniformlydistributed over the irradiated area. In other words it may in this casebe desirable that the intensity is essentially equal in all directions.

[0014] In addition, sharp edges or corners should be avoided becausethese might accidentally damage the vessel or tissue.

[0015] By using the manufacturing method according to the independentmethod claim a further object may also be achieved, namely a possibilityto customize the X-ray source with regard to radiation distribution.

[0016] Thus, the manufacturing method according to the present inventionis advantageous in at least two aspects: it is a cost-efficientmanufacturing method of a large number of X-ray sources and it makes itpossible to customize the X-ray source with regard to radiationdistribution.

SHORT DESCRIPTION OF THE APPENDED DRAWINGS

[0017]FIGS. 1a-1 c illustrate schematically the structure of a miniatureX-ray source manufactured by using the method according to the presentinvention.

[0018]FIGS. 2a-2 b illustrate schematically a part of the manufacture ofan X-ray source having a hexagonal outer shape.

[0019]FIG. 3 illustrates schematically a part of the manufacture of anX-ray source having an octagonal outer shape.

[0020]FIG. 4 shows in a graph the difference in absorbed dose in vesselwall vs. distance into the vessel wall for a square X-ray sourcestructure where the dose is normalized to 100% at 0.7 mm for thick wall.

[0021]FIG. 5 shows in a graph the difference in absorbed dose in vesselwall vs. distance into the vessel wall for a hexagonal X-ray sourcestructure where the dose is normalized to 100% at 0.7 mm, for thickwall.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0022]FIG. 1a schematically illustrates a miniature X-ray source in across-section, generally designated with reference numeral 2. Itcomprises a support structure 4 in which an X-ray cavity 6, providedwith vacuum, is defined by an anode 10 and a cathode 8 at opposite sidesof the cavity. The anode is connected to the positive pole of a highvoltage source (not shown), and the cathode is connected to the negativepole.

[0023] In FIG. 1b the device of FIG. 1a is shown schematically fromabove. As can be seen it has a square outer shape. This comes from thestandard manufacturing techniques used in semiconductor technology.Namely, starting from a e.g. circular wafer 12 of a suitable material(see FIG. 1c), processing the wafer according to a number of processsteps such as lithographic methods, etching, deposition etc, to comprisea large number of discrete elements, schematically indicated byreference numeral 14. When the manufacturing process for the discreteelements is completed, sawing along the lines 16 indicated in FIG. 1ccuts out each element.

[0024] For miniature X-ray sources the radiation strength depends uponthe level of attenuation in the support structure. The attenuationdepends, among other things e.g. the material of the structure, and thedistance from the center of the X-ray source the radiation has totravels through the support structure. FIG. 1b schematically illustratesdifferent distances X1 and X2 in a square-shaped support structureresulting in a lower radiation dose beeing delivered in the direction ofX1 than in the direction of X2 due to the higher radiation attenuationin direction X1.

[0025] Many different materials may be used, e.g. alumina(polycrystalline Al₂O₃), sapphire (crystalline Al₂O₃), pyrolytic orcubic boron nitride (BN) or quarts (SiO₂). When determining whatmaterial to be used the electrical break-through voltage is of greatconcern. Namely, the thickness of the material must be dimensionedtaking account the thinnest part of the structure.

[0026] Pyrolytic boron nitride is advantageous with regard to that ithas an acceptable electrical break-through voltage.

[0027] It is especially advantageous to choose a material having an inintrinsic crystal structure that is the same as the cross-sectionalshape of the support structure. Both pyrolytic boron nitride andsapphire have a hexagonal crystal structure and by dividing the wafer inhexagonal-shaped structures in the directions that correspond to thecrystal structure the intrinsic material characteristics are optimallyused.

[0028] According to a first aspect of the invention a uniformlydistributed radiation around the X-ray source is desired, i.e havingessentially the same level of radiation in each direction in a planeperpendicular to the direction of a through-going hole in the supportstructure.

[0029] According to a preferred embodiment embodying this first aspectof the invention the ideal cross-sectional shape of the cavity iscircular and the cross-sectional shape of the support structure is alsocircular. These circles are concentric.

[0030] However, in order to be able to manufacture large numbers ofX-ray sources, the inventor has found that manufacturing techniques usedin the semiconductor industry is also applicable for manufacturing X-raysources.

[0031] Although possible to divide a wafer into circular elements it isconsidered more cost-efficient to divide the water along straight lines,preferably by using sawing techniques.

[0032]FIGS. 2a and 3 show two different alternative shapes of thecross-section of support structure that may be obtained by straight-linedivisions of a wafer.

[0033] The holes made in the wafer to provide the X-ray source cavitiesmay be achieved in two principally different ways.

[0034] According to a first preferred embodiment the cross-section ofthe cavity is to have a circular shape the most convenient method is toprovide a wafer of a suitable material, e.g. alumina, and to make holesby laser drilling. FIG. 2a shows a part of a wafer 18 having holes 20made by laser drilling. When the holes are made, cathodes 22 and anodes24 can be mounted in the holes by a suitable bonding method orsoldering, see FIG. 2b, which is a cross-section through the wafer inFIG. 2a. Then the wafer is divided as shown in FIG. 2a along lines 21 inorder to provide X-ray sources having a hexagonal cross-section.

[0035] According to a second preferred embodiment the cross-section ofthe cavity is to have a polygonal shape, e.g. hexagonal or octagonal,the holes are made already when the wafer is manufactured. Namely, apositive mold is provided, having protrusions of the desired geometry(polygonal, e.g. octagonal or hexagonal). A ceramic paste is spread onthe mold and exposed to sintering conditions. When the mold is removedthe wafer will be provided with holes of the desired geometry. In thecase of alumina, the wafer may be precision machined (e.g. by drilling,milling, etc.) before sintering.

[0036] Then, again, cathodes and anodes, shaped as to fit in the holes,are mounted in the holes so as to seal the cavity, and the wafer isdivided to produce the hexagonal shaped support structures. The anodeand cathode structures may also be made as structures on entire wafers,if properly designed with respect to for instance thermally inducedstress.

[0037] If an octagonal, cross-sectional shape 26 of the X-ray source isdesired, the dividing pattern will be as shown in FIG. 3. Obviously thisdividing pattern will yield much more waste material, but it may bejustified by the shape obtainable being somewhat better suited for thepurpose of uniform radiation distribution than the hexagonal dividingpattern.

[0038] When the elements have been made is described above, such thatthey comprise a support structure of e.g. alumina, and an anode and acathode are mounted in the hole to form the cavity, the X-ray source iscompleted as follows.

[0039] The cavity must be evacuated, which can be achieved with methodsknown in the art such as by employing evacuation channels and gettermaterials, and will not be further discussed herein.

[0040] According to a preferred refinement of the invention theindividual elements are embedded in a polymer by an injection moldingtechnique, so as to form a “bucket” of plastic around the element.Conveniently the mold has a cylindrical shape so as to produce an X-raysource having a generally tubular shape, which is most suitable forinsertion into blood vessels which have a generally tubular geometry.However, any shape can be made if desired.

[0041] The invention will now be further exemplified by comparisonsperformed with a square configuration and a hexagonal configuration,respectively.

[0042] The following has been used for the examples below: The targettissue is assumed to be located 0.7 mm. into the vessel wall. Thistarget must receive the prescribed dose. If the radiation emitted fromthe anode will be asymmetrically absorbed, the maximum absorption in thematerial will occur where the support structure wall thickness islargest. This will in turn give an overdose to the tissue that isexposed to radiation travelling through the support structure wall wherethe thickness is smallest. Furthermore, the over dose to the vessel wall(x=0) will also change. A typical prescribed dose is 15-18 Gy. at 0.7mm. into the vessel well. The overdose to the surface should not exceed50 Gy.

[0043] In addition, a minimum support structure wall thickness of 200 μmhas been used, because it has to be thick enough to be mechanicallystable (not to break) and has to be vacuum tight enough. Thinner wallsare possible but will be more brittle and more sensitive to gaspermeation. The structure is assumed to fit into a 1.5 mm. diametercircle.

[0044] Mass attenuation data are taken from the National Institute ofStandards and Technology. A typical vessel diameter of 3 mm, has beenused, assuming a centered source. The radiation has been assumed to be abrehmsstrahlung spectra energized at 20 kV.

EXAMPLE 1

[0045] (Calculation of Absorbed Dose From a Square Geometry Element in a3 mm Vessel, in Different Directions):

[0046] Difference in max/min circumferentially absorbed dose:

[0047] Target tissue dose: 183% (Max dose 33 Gy if prescribed dose is 18Gy)

[0048] Vessel surface dose: 337% (Max dose 61 Gy if prescribed dose is18 Gy)

[0049]FIG. 4 shows the difference in absorbed dose in a graph.

EXAMPLE 2

[0050] (Calculation of Absorbed Dose From a Hexagonal Geometry Elementin a 3 mm Vessel, in Different Directions):

[0051] Difference in max/min circumferentially absorbed dose:

[0052] Target tissue dose: 134%. (Max dose 24 Gy if prescribed dose is18 Gy)

[0053] Vessel surface dose: 248% (Max dose 45 Gy if prescribed dose is18 Gy)

[0054]FIG. 5 shows the difference in absorbed dose in a graph.

[0055] In addition the treatment time will be 42% longer when using asquare geometry, as compared to a hexagonal geometry.

[0056] The above examples are somewhat simplified for clarity. Forexample a biocompatible coating must be used and, this has not beenincluded in the calculations.

[0057] According to a second aspect of the present invention thepresumption is that a non-uniformly radiation distribution from theX-ray source is desired. This may e.g. be the case if an identifiedtreatment site is located at one side of a blood vessel.

[0058] By optimizing the form of the support structure a radiationwindow may be arranged that exhibits the highest radiation dose. This isachieved by varying the thickness of the support structure wall suchthat the smallest thickness is arranged where the highest radiation doseis desired, and thereby defining the position of the radiation window,and the largest thickness is arranged where the highest attenuation iswanted.

[0059] The present invention is not limited to the above-describedpreferred embodiments. Various alternatives, modifications andequivalents may be used. Therefore, the above embodiments should not betaken as limiting the scope of the invention, which is defined by theappending claims.

1. Miniature X-ray source comprising a support structure (4,26) providedwith a through-going hole (6,20), an anode (10,22) is arranged at oneend and a cathode (8,24) at the other end of the hole, thereby defininga cavity, the anode and cathode are adapted to be energised in order togenerate X-ray radiation, characterized in that the support structurehas a cross-sectional shape that is determined such that a desiredradiation distribution of the radiation generated by the X-ray source isachieved.
 2. Miniature X-ray source according to claim 1, characterizedin that said support structure is provided with a wall having a wallthickness between the hole and the outer side of the structure, whereinthe radiation distribution depends of said wall thickness.
 3. MiniatureX-ray source according to claim 2, characterized in that if a uniformradiation distribution is desired the support structure has an even wallthickness.
 4. Miniature X-ray source according to claim 2, characterizedin that if a non-uniform radiation is desired the support structure hasan uneven wall thickness.
 5. Miniature X-ray source according to claim1, characterized in that the support structure is made from a wafer fromwhich a large number of support structure for X-ray sources areobtainable.
 6. Miniature X-ray source according to claim 1,characterized in that the cross-sectional shape of the hole isessentially the same as the cross-sectional shape of the supportstructure.
 7. Miniature X-ray source according to claim 1, characterizedin that the cross-sectional shape of the hole is circular.
 8. MiniatureX-ray source according to claim 1, characterized in that thecross-sectional shape of the support structure is polygonal. 9.Miniature X-ray source according to claim 1, characterized in that thecross-sectional shape of the support structure is hexagonal. 10.Miniature X-ray source according to claim 1, characterized in that thecross-sectional shape of the support structure is octagonal. 11.Miniature X-ray source according to claim 1, characterized in that thecross-sectional shape of the support structure is circular. 12.Miniature X-ray source according to claim 2, characterized in that apart of the wall has a wall thickness with lesser thickness than therest of the wall thereby defining a radiation window where radiation ata higher dose is adapted to be provided.
 13. Method of manufacturingminiature X-ray sources comprising the following steps: i) makingthrough-going holes, one for each X-ray source to be manufactured, in adisc-shaped support structure wafer having an even thickness, ii)arranging for each hole an anode and a cathode at opposite sides of thewafer and thereby defining an X-ray source cavity between the anode andthe cathode, iii) dividing the wafer into separate elements where eachelement includes an X-ray source and where the support structure of eachX-ray source has a predefined outer shape that is determined such that adesired radiation distribution of the radiation generated by the X-raysource is achieved.
 14. Method according to claim 13, characterized inthat the holes made in step i) is made by laser drilling.
 15. Methodaccording to claim 13, characterized in that the holes made in step i)is made during manufacture of the wafer, preferably by moulding thewafer.
 16. Method according to claim 13, characterized in that the holesmade in step i) is made during manufacture of the wafer, preferably byprecision mashining.
 17. Method according to claim 13, characterized inthat in step ii) the cavity is evacuated.
 18. Method according to claim13, characterized in that in step iii) the wafer is divided by a sawingoperation.
 19. Method according to claim 13, characterized in that instep iii) the wafer is divided by a laser operation.
 20. Methodaccording to claim 13, characterized in that in step iii) the wafer isdivided by a blasting operation.
 21. Method according to claim 13,characterized in that in that step iii) the wafer is divided by ascribing and cracking operation.
 22. Method according to claim 13,characterized in that in step iii) the wafer is divided by a usingpreformed scribeline followed by a cracking operation